The present invention relates generally to power distribution systems and, more particularly, to power distribution systems for offices and other environments in which power is supplied to a large number of computers or other pulsed, non-linear electrical loads.
Office power distribution systems supply electrical power to a variety of single phase and three-phase electrical loads. Typical loads have, in the past, included motors, lighting fixtures, and heating systems. These loads are, for the most part, linear in nature. When an alternating current is applied to a linear load, the current increases proportionately as the voltage increases and decreases proportionately as the voltage decreases. Resistive loads operate with a power factor of unity (i.e., the current is in phase with the voltage). In inductive circuits, current lags voltage by some phase angle resulting in circuits which operate with a power factor of less than one. In a capacitive circuit, the current leads the voltage. However, in all of these circuits, current is always proportional to the voltage and, when a sinusoidal voltage is applied to the load, the resulting current is also sinusoidal.
Until recently, almost all loads found in a typical office environment were linear loads. However, computers, variable speed motor drives, and other so-called "electronic" loads now comprise a significant and growing portion of the electrical load present in offices. These electronic loads are, for the most part, non-linear in nature. These loads have become a significant factor in many office power distribution systems, and their presence has lead to a number of problems and office power system malfunctions.
A non-linear load is one in which the load current is not proportional to the instantaneous voltage and, in many cases, is not continuous. It may, for example, be switched on for only part of a 360 electrical degree alternating current cycle.
The presence of non-linear loads on a power system can cause numerous problems. Typical office power distribution systems operate as three-phase 208/120 volt systems with a shared neutral conductor serving as a return path for currents from each of the three phases. Linear loads which are balanced among the three phases produce currents which typically cancel in the shared neutral conductor resulting in relatively low net current flow in the neutral. Pulsed currents produced by non-linear loads do not cancel in the neutral conductor because they typically do not occur simultaneously. These currents tend to add on the neutral even when the three phases of the system are carefully balanced. The resulting high current flows in the neutral conductor can lead to severe overheating or burnout of neutral conductors, and increased noise levels on the neutral. Pulsed, non-linear currents further cause relatively large variations in the instantaneous power demanded from the generator. These variations can cause problems and inefficiencies on the generator and distribution side of the transforming device. Moreover, pulsed, non-linear currents may cause typical induction watt-hour meters to show large calibration errors.
An object of the present invention is to provide a power distribution system for an office environment in which the adverse affects of pulsed, non-linear loads are reduced.
This object is achieved in a power distribution system in which three-phase electrical power is supplied to a primary side of a power transforming device, and in which at least six phases and a shared neutral conductor are provided at the secondary side of the transforming device. A plurality of electrical loads, including non-linear loads, are distributed between each of the six phases and the shared neutral so as to reduce by current cancellation the current which would otherwise flow in the shared neutral conductor due to the presence of the non-linear loads. Each of the first, second and third of the six phases provided at the output of the transforming device are preferably separated from each other by 120 electrical degrees. The fourth, fifth and sixth of these phases are also preferably separated from each other by 120 electrical degrees, and are from the first, second and third phases, respectively, by 180 electrical degrees. In a particularly preferred embodiment of the invention, at least 12 phases are produced at the secondary side of the power transforming device. Each of these 12 phases is preferably separated from the other phases by 30 electrical degrees In this embodiment, the 12 phases may be viewed as two sets of six phases, with each of the phases in a first of the two sets shifted relative to respective phases of the second set, so as to reduce variations in the level of instantaneous power drawn from the input source which would otherwise occur due to the presence of the non-linear loads. In this preferred embodiment, the six phases in the first set are preferably shifted by 30 electrical degrees relative to respective ones of the six phases in the second set. This may be advantageously accomplished by shifting each set of six phases by 15 electrical degrees in opposite directions relative to the phase angles of the incoming power source.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.